How Do Guitar Tube Amps Work? – Our Comprehensive Guide

A detailed guide on how vacuum tube technology works, components and schematic of the tube amps, and useful tips on how to replace a bad tube.
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Last updatedLast updated: August 30, 2021
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Even though vacuum tubes are technology from the 30s and 40s, the best guitar amps today still use vacuum tubes despite the availability of more modern technology. Amps with vacuum tubes simply sound too good to be replaced by anything else as yet.

“How do tube amps work?” This lesson will take you back to the fundamentals of vacuum tube technology, the components and assembly of tube amplifiers, and everything else you need to know to make even better use of them.

Before we start poking around at the parts of a tube amplifier, we must sound a warning. Tube amps come from way back when everything was trying to kill you. Some of their internal components operate on several hundred volts (450 or more) and contain some seriously lethal capacitors.

In other words, don’t touch anything inside your tube amplifier if you don’t know what you’re doing. With that, let’s start with an overview of vacuum tube technology.

Tube Amp Overview

Tube amps work because vacuum tubes have excellent audio characteristics. When it comes to amplifying signals, inverting their polarity, or splitting and mixing signals, vacuum tubes (also called valves) perform outstandingly well and add a unique personality to the result.

What is a Vacuum Tube?

How Do Guitar Tube Amps Work? - Our Comprehensive Guide

A vacuum tube is an electrical component made of a tube with the air sucked out and fitted with various electrical components inside. They work because electrons can travel through space in a vacuum, while they cannot do that easily through non-conductive air (it does happen when high voltage lines cause arcing).

The whole purpose of a vacuum tube is to control the flow of electrons. Some of the components you will find inside the tube include a cathode, heating element, grid, and plate.

Tube Diagrams

The basic design of a vacuum tube hasn’t changed in over a century. They consist of a cathode (electron emitter), anode, grid (with the input signal), as well as a few more components depending on the desired purpose.

Here is a simplified diagram showing the internal components of a vacuum tube:

How Do Guitar Tube Amps Work? - Our Comprehensive Guide

And here is a more detailed image of a vacuum tube showing the internal components. We will have a look at what each of these parts does in the assembly.

How Do Guitar Tube Amps Work? - Our Comprehensive Guide

The added parts in this image are metal grills used to control the flow of electrons and reduce capacitance, thus improving the performance of the tube immensely.

The most basic design of a vacuum tube mainly consists of four

  • Cathode

A cathode is a positively charged pole. It is an element with a slight positive charge located at the centre of the tube, and which emits a stream of electrons (negative charge) in the right conditions. These electrons pass through the vacuum to a second plate with an even stronger positive charge.

One of these “right conditions” is the presence of another plate with a much stronger positive charge (the anode). This difference in charge attracts a high flow of electrons because unlike charges tend to attract. With the right control, this flow can be used to manipulate a signal.

  • Heating Filament

The second condition necessary to ensure the flow of electrons is heating. The cathode needs to be heated so that electrons can flow freely. The filament will be located next to the cathode in the centre to facilitate this; alternatively, the element itself will be the cathode and coated with special material to provide the flow of electrons.

  • Plate (Anode)

A plate with a high positive charge surrounds everything else inside the vacuum tube. It works by attracting the negatively charged electrons towards itself inside the vacuum tube, which makes it the anode. It collects the electrons emitted by the cathode at the centre and also picks up the signal from your guitar pickup.

  • Grid

So far, we haven’t talked about how the vacuum tube amplifies the signal from your guitar pickup. This is where things get really nifty.

A fourth component called a grid comes between the plate and the cathode. The grid is a piece of metal connected to the input from your guitar, which charges it and gives it a small positive or negative charge depending on the incoming signal.

This is why the cathode is not really negatively charged but instead has a slightly positive charge. Without any current flowing in it, the grid will be negative by comparison, which helps to repel the electrons in the anode and keep them in place until a voltage is applied to attract them.

What happens when you play the guitar is that your pickup applies a very small voltage (1-4V) across the grid, which fluctuates in tune with your playing. This small signal attracts a large number of electrons from the anode, which the plate collects as an amplified signal.
The interaction of electrons in the tube also creates natural harmonics, which is what gives tube amps their unique sound distortion.

Obviously, this process is far more complex than this, but you can now understand the basic operation of a vacuum tube. There are other components in a tube amp that work together with the vacuum tubes to become the kind of amps loved by the best musicians everywhere.

Components of a Tube Amp

How Do Guitar Tube Amps Work? - Our Comprehensive Guide

There are a few more principles of electricity that you have to understand that play a part in the operation of a tube amp as a whole. First, the relationship between current, voltage, and resistance is defined as:

V (voltage)  = I (current) x R (resistance)

In other words, the voltage is the product of the current and resistance measured between the ends of a conductor or circuit. If you think of current as the amount of water flowing through a pipe, voltage is the pressure pushing the water and resistance is caused by any restriction to this flow.

Secondly, mains electricity is supplied as alternative current (AC) that changes direction 50-60 times every second. This electricity comes in the form of a sine wave, but audio equipment such as tube amps operate on a steady DC current.

Thus, a tube amp will have a device to convert mains electricity from AC to DC, then a transformer steps it up and down as required to supply it at different voltages to the other components in the system. Let’s take a close look at each of these components that make up a tube amplifier.

Power Transformer

While the tubes in the circuit operate at over 200V, the elements in the valves require 6.3V AC current to work. Thus, the first step is to step up or step down the electricity supply as required and send it to the different circuits.

Rectifier/Diode

The rectifier valve converts (or rectifies) the AC signal from the supply to a DC signal that the tubes can use. In older amps, this was also done using a tube (valve) rectifier, but silicone diodes have since replaced them in more modern equipment.

The different electrical waveforms look like this:

How Do Guitar Tube Amps Work? - Our Comprehensive Guide

As you can see, an alternating current represented by the green line has a sinusoidal (up and down) waveform, while we need the more stable direct current. Because the waveform keeps changing direction, the rectifier works by correcting this reversal to produce a unidirectional waveform.

Capacitors

The rectified DC waveform is not completely straight. Instead, it tends to look more like the pulsating waveform above. Capacitors act like dams on a river, storing charge and releasing it in a more steady form and smoothing out the “ripples” in the flow.

These capacitors continue to store a lethal dose of charge even when the tube amp is switched off. Due to the high voltages involved, this charge can be lethal if you touch the wrong place and they dump this charge inside your body. That’s why only trained technicians should open up tube amps.

The high DC voltage produced by the capacitors then goes to the plates in the tubes, which require a high positive potential to attract electrons from the cathode. If further voltage or current regulation is required down the line, it is accomplished by the use of resistors.

Resistors

Like a constriction in a pipe, resistors can be used to control and reduce the amount of voltage and current in a system. They dissipate electrical energy depending on their resistive value, whose units are ohms.

If you’re interested to know, older tube amps had more character and personality because they used older resistor technology. Carbon-comp resistors added “noise” to the system, which produced the vintage sound we know so well. Some modern manufacturers use “clean” metal film resistors that give a truer signal but with less character.

If you ever build your own tube amp or would like your technician to help you experiment a bit, you can always suggest changing the resistors in the output stages of the circuit.

Preamp Tubes

How Do Guitar Tube Amps Work? - Our Comprehensive Guide

Many valve amplifiers have two sets of tubes: preamp tubes and power tubes. The preamp tubes are the first set of tubes that receive the guitar pickup signal. Their work is to pre-amplify the guitar signal to a level that can be applied on the larger power tubes, and they also drive reverb or tremolo effects.

The 12AX7 type of preamp tubes is one of the most common. The 12 in the specification indicates the amount of heater voltage they take (as opposed to 6.3V), but different manufacturers make them in very different ways. The most common 12AX7 tubes have a gain factor of 100, which makes them one of the most powerful options available.

The most important job of preamp tubes is to give character to the input signal. They add rich harmonics, create distortion, and add sustain to the sound at high volumes; this is known as overdriving the preamp.

Preamp tubes mostly contain the three parts described above: cathode, grid, and anode. This is known as a triode design.

 Power Tubes

How Do Guitar Tube Amps Work? - Our Comprehensive Guide

Power tubes are a much larger type of valve which takes the preamp signal and amplifies it to speaker levels. (on the other hand, preamps amplify signals to line level). They are usually five-part designs or pentodes, adding more screens to the three major components to help with electron flow control.

Examples of pentode power tubes are the EL84s and EL34s, whose performance is almost comparable to that of solid-state components. The power tubes are controlled by the master volume dials to vary the volume levels, but they don’t color the sound output as much as the preamp tubes.

You can easily swap out both the preamp tubes and power tubes to experiment with those from different manufacturers. This gives you the freedom to play about with different types of sound characteristics from a clean sound to all-out distortion, according to your preferences.

However, be careful when swiping out the tubes. Never swap out hot tubes, and be careful to follow all manufacturer guidelines while doing so. Never force a tube in or out, and always have a technician do this for you if the chassis (cover) needs to be removed to change the tubes.

Vibrato Oscillator

One of the ways that tube amps can add color and texture to sound is through the use of a vibrato oscillator. Actually, the correct technical term is a tremolo oscillator, but Fender uniquely referred to it as vibrato in the ’60s and it stuck.

Tremolo is a form of slow amplitude modulation, an effect where a low-frequency wave (5Hz-10Hz) is used to alter the audio signal in frequency and amplitude. The wave itself is inaudible, but the effects it causes on the output sound waves are amazing.

Tremolo is made with sine or triangular waves, which means another circuit must be added to create and control these waves. An amp or pedal device for tremolo is needed to alter the speed and depth of the alterations, controlling the linear rise and fall in the sound signal.

Output Transformer

The output transformer takes the signal from the power tubes and converts it into the right impedances and voltage for the speakers. Given that the tubes output hundreds of volts, which definitely would blow the speakers if connected directly. The output transformer transfers the signal load without any major alterations to the tone and color of the sound output.

Amplifier Operation

How Do Guitar Tube Amps Work? - Our Comprehensive Guide

Amplifier operation can be classified either as Class A, Class B, or Class AB.

  • Class A tubes run the full sine wave from the guitar input. They have a positive signal applied to the grid, and the current is flowing at all times when the unit is on
  • Class B amps use multiple valves or transistors that share the different phases of the sine wave. In other words, they conduct only one-half of the input signal each, which is why they are also called push-pull amps
  • Class AB tubes handle more than half the wave, but not the whole wave. They have a negative “bias” voltage applied to the grid so that the circuit turns off if the audio waveform goes below a certain point. It will have a different tube or circuit turning on at such times, thus having the unit operational at all points
Tube amps with one power tube are always Class A because the tube must always handle the full sine wave from the input signal. However, most tube amps have four valves instead of just one.

The four tubes are placed in two sets of A-B operation tubes so that they add power to each “half” of the sine wave. This allows them to work like a single, more powerful tube. There are advantages and disadvantages to each of these common modes of operation.

Class A Operation

Advantages

  • The tube is always ready to amplify the signal. It does not lag by needing to “wake up” from a state of not being operational
  • They sound louder than class AB tubes of the same power because the is no signal splitting
  • They have smooth compression because the current is always at maximum
  • The amp feels more responsive and smooth because it performs instantaneous amplification

Disadvantages

  • Since the current is always on, the tubes are strained even when you are not playing
  • The tubes handle a full load, so they have a shorter life
  • They require a more powerful transformer to handle the higher current

Class AB Tubes

Advantages

  • The tubes share the current and signal, giving them some “idling” time. This gives them a longer lifetime
  • They tend to have a tighter bass response
  • The transformer can handle class AB tubes better because they are less demanding

Disadvantages

  • They are less responsive than single Class A tubes

There are other classes of amplifiers as well such as Class G and Class H, but they are not used for audio applications.

Talking of Fender, the company still makes the best tube amps that are favored by professional musicians and music directors the world over. The Fender Pro Junior IV is an especially popular part of the “Hot Rod” series released in 1996. Featuring high-end, all-tube performance, it is a highly portable unit with two speaker outputs and an 8-ohm primary tap. It is a vintage beauty with a warm and butter sound that you simply can’t resist.

The Tube Amp Schematic

Audio Tinker has a simplified schematic showing how everything comes together in a tube amplifier. If you are not familiar with the symbols, the labeling will help you figure things out.

  • R with the zigzag symbol represents resistors
  • The upside-down tree of parallel lines represent ground – connection to chassis
  • C with the heavy = sign represents the capacitors
  • The small rounded parts are preamp tubes, which the big oval part is the power tube
  • The rectangle with four arrows represents the rectifier unit. The arrows are diodes
  • The adjacent coils marked PT are potentiometers, in other words, these are the dials used to control gain, volume, and other features

How Do Guitar Tube Amps Work? - Our Comprehensive Guide

You can check out the full instructions on the AudioTinker website to learn more about the circuit, assembly, and specifics of the components used.

Tube Amps Vs. Solid-State Amps

Now that you have a basic idea of how tube amps work, it is important to compare their performance with that of solid-state amplifiers.

Solid-state amplifiers amplify signals using transistors and diodes. You would expect the more modern technology to produce better results, but that isn’t the case. Tube amps are warm and produce a pleasant, full-bodied result. In more technical terms, tube amps produce audile with a good bass and midrange response.

When you overload a tube amp, the effect is even more pronounced. They cause a distortion of sorts that makes the sound even better, especially when playing loud guitar music in genres like rock and roll. If you play heavy metal and rock, take a look at these top 8 amps for metal you can buy today.

In comparison, solid-state amps are more faithful to the original signal and produce a cleaner sound. Because they don’t need to be overloaded, they have more headroom (loudness without distortion), so they tend to be popular with bassists, keyboard players, and in jazz.

An amplifier without natural harmonic distortion sounds unnatural, however, so some modern amps called modeling amps tend to mimic the sound of tube amps.

Comparison: Advantages and Disadvantages

Tube Amps Advantages

  • They produce a warmer, naturally pleasant sound due to harmonic distortion
  • Tube amps can get really loud
  • Even at high volumes, tube amps are smooth, warm, and better-sounding

Tube Amp Disadvantages

  • Tube amps are fragile and require a lot of care in set up and maintenance
  • They are very expensive to buy and maintain
  • Using tube amps requires significant technical knowledge because they are less forgiving of mistakes in setup and equipment matching
  • Tube amps are large and heavy, which makes transportation a nightmare
  • Not as many manufacturers are making tube amps anymore, or even their spare parts

Solid-State Amps Advantages

  • Solid-state amps are much cheaper to buy and maintain
  • They require less technical knowledge to maintain; no tubes to swap out, and no yearly upkeep required
  • Solid-state amps are durable because they have no glass tubes to think about
  • They sound much better and cleaner at low volumes. Tube amps, on the other hand, work best when you crank them up to overdrive
  • They contain more complex circuitry, which makes them more versatile for extra effects. This reduces the need to add foot pedals for extra effects

Solid-State Amp Disadvantages

  • They lack the mellow, desirable effect of harmonic distortion for which tube amps are so popular. Their pure, clean sound can even sound unnatural
  • Even the most modern modeling amps aren’t as good as pure tube amps

Hybrid Amps

On paper, it would seem that solid-state amps beat tube amps squarely. However, the warm and mellow sound produced by tubes makes them worth all the extra expense and effort required to use them.

Today, most amps are actually hybrids of both technologies. They have a tube preamp stage to

add the required coloring and distortion, but solid-state circuitry for the power stage of the amplification. They aren’t as warm as the true tube amps, but the tradeoff in convenience and ease of use makes them very popular in recording studios.

How To Know a Tube Is Bad

Tubes require regular checking and swapping depending on the frequency of playing. That could be anything from 6-month intervals to a couple of years.

Vacuum tubes wear out naturally with time. They can simply burn out, or gas can form inside the tube and make them less effective. This causes them to become less effective. Here are some signs of bad tubes.

  1. A loud squealing sound even without having a guitar plugged in, which is usually caused by one faulty tube. Restart the amp, identify the culprit, and replace it immediately.
  2. Excessive hissing or humming noise.
  3. Loss of high-end treble.
  4. Excessive bass with no clarity, which creates a muddy sound.
  5. Erratic volume changes.
  6. Blown fuses.
  7. Rattling inside the valve.
  8. Discoloration on the valve glass cover.
  9. Tap each valve when the amp is on but no guitar is plugged in. Any zings, pings, or crackles indicate time for a replacement.

When you have a faulty valve, it is advisable to replace all of them at once. This ensures correct biasing and restores full performance. You should always have replacement valves with you in case of failure because sometimes playing with a bad one can damage the other internal components.

This should be a problem, however. You can buy these Genalex premium preamp tubes for less than $50 a pop. They are designed to resist feedback and make a lively upgrade to your old tube amp, and are some of the most popular 12AX7 preamp tubes out there.

How Do Guitar Tube Amps Work? - Our Comprehensive Guide

How to Replace a Bad Tube

Once you have determined that you have a bad vacuum tube, the next step is to remove and replace your tubes following the recommended safety guidelines. The manufacturer’s instructions always take precedence in this case, but here are a few pointers.

  1. Unplug your amp and leave it to cool down completely. The tubes usually run very hot, so give them enough time to cool down.
  2. If the tubes have a cover, unscrew them. If you need to remove the entire chassis to reach the tubes, let a qualified technician replace the tubes for you. They will know how to handle the high-voltage capacitors safely.
  3. Grasp the top of each tube and gently ease it out from its slots. Use a gentle twisting motion to free it if it feels stuck.
  4. Remember not to use too much force, because the glass top can crack and break easily in your hand.
  5. Once the old tube is off, take a corresponding replacement and line it up with the slots.
  6. Gently push, rock, and twist the tube in place, being careful not to break or crack it.
  7. Repeat for all the tubes you need to replace. If you have changed the type of power tubes, they may need rebiasing by a qualified technician. This is especially important if you have different models, which need biasing to run consistently.

FAQs

  • Can you replace the tubes in your guitar amp yourself?

Yes, most manufacturers account for this in their design. The tubes are located outside the chassis with a different cover, and the tubes themselves won’t require rebiasing so long as you use recommended replacements.

  • How long does a tube amp valve last?

The vacuum tubes in a tube amp generally last 5,000 to 10,000 hours of play time. However, this depends on the intensity of use. The best way to know when it’s time for replacement is to check for any strange sounds or lag in performance.

  • Are tube amps worth it?

Yes, tube amps certainly sound better than solid-state amps and any other alternative technology. Even without being a purist, you will hear the difference in the warmth and character of the sound they make compared to the cold, clean sound of solid-state amps.

  • Should you turn off your tube amp when not using it?

Yes. Vacuum tubes wear out with use, so it is better to always turn off your amp when you are done using it. They also tend to overheat and use up a lot of power, so it is safer and cheaper to have them off until they are needed.

  • Do you really need a tube amp?

Despite the amazing sound that tube amps produce, the answer here is “no, not really.” The full character of tube amps only comes out when they are used to the max, which means in recording or performance sessions.

If you just need an amp for practice or move around a lot, a tube amp is not worth the money or effort required. You might want to check out our top five bass practice amps for this instead.

  • Do tube amps need to warm up?

Yes, tube amps need about 20-30 minutes to warm up properly before you can start playing on them. Your manufacturer will have provided guidelines on this.

  • How do you tone a tube amp?

To take maximum advantage of overdrive, you should crank the volume on the amp all the way up and then use your guitar’s pickup controls to control the overall volume. This will give you maximum character even at relatively lower volumes.

  • How often do you need to service a tube amp?

Apart from replacing the tubes as needed, a good tube amp won’t need service or repair for 10 years or more. It is always advisable to buy high-quality tube amps from recognized manufacturers in the first place, such as Fender or Marshalls.

Final Thoughts

Century-old technology still trumps the most advanced transistor and microchip technology when it comes to amplifying guitar music. The natural harmonic distortion that tube amplifiers produce is what makes them worth all the effort required in maintaining and using them.

Hopefully, now you understand how tube amps work from this simplified explanation. The amps themselves are so simple that you can build one yourself given the right parts, but the technology behind them is simply ingenious.

You can also now take better care of your old tube amp, and perhaps even perform a few minor upgrades to get the best sound. Remember to enjoy the mellow sound of these antiquated units before impostor technology sweeps them all to the graveyard.

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